The present invention relates to a liquid crystal display device and to an optimum gradation voltage setting apparatus, which determines γ characteristics. The present invention is also applicable to display devices having optical γ characteristics in general, such as PDP and EL display devices, besides the above-mentioned liquid crystal display device.
FIG. 8 is a block diagram which schematically shows one examples of the circuit constitution of a conventional liquid crystal display device (liquid crystal display module) which has been developed by the inventors of the present invention. The liquid crystal display device is constituted of a liquid crystal panel 1, a gate driver section 2, a source driver section 3, a display control circuit 4, a power source circuit 5, a ladder resistor circuit 8 and a driver circuit 9, wherein the liquid crystal display device is operated when digital signals, such as display data signals, synchronizing signals and the like, are inputted as input signals, and input power is supplied to the power source circuit 5.
The inputted digital input signals are temporarily subjected to a timing adjustment using synchronizing signals and the alternation of the display data signals in the display control circuit 4, such that a direct current is not applied to the liquid crystal panel, and these synchronizing signals and the display data signals are transmitted to the gate driver section 2 and the source driver section 3.
Further, the gradation voltage from the driver circuit 9, to which a voltage divided by the ladder resistor circuit 8 is inputted, is transmitted to the source driver section 3, while the source driver section 3 selects the gradation voltage corresponding to a level of the display data signal and supplies the gradation voltage to the liquid crystal panel 1, whereby the display data signals are displayed on a screen of the liquid crystal panel 1.
Here, the source driver section 3 further divides the gradation voltages, thus outputting 64 to 256 kinds of voltages in total, whereby a fine gradation display can be produced.
FIG. 9 is a diagram of a conventional circuit for generating gradation voltages, which shows an example of the ladder resistor circuit 8 and the drive circuit 9 of FIG. 8. In the drawing, the ladder resistor circuit 8 divides the potential difference between the reference voltage 1 and the reference voltage 2 using resistors 10, determines voltages in the form of required gradation voltages 1 to n, and supplies these voltages to the source driver section 3 through buffers 11 in the drive circuit 9.
FIG. 10 is a graph which shows the relationship between the gradation voltage and the brightness. Although the relationship usually exhibits the standard brightness characteristic 12, due to various factors, such as irregularities in the manufacture of the panels, a temperature change or the like, the brightness characteristic 12 may be shifted in the up-and-down direction with respect to the brightness and shifted in the left-and-right direction with respect to the gradation voltage, thus exhibiting one of the brightness characteristics 13. With respect to the gradation voltage m, since the brightness Bm corresponding to the gradation voltage is shifted in the up-and-down direction by ±ΔB, for example, the γ characteristic, which represents the relationship between the input signal and the brightness, as shown in FIG. 11, is shifted, thus causing the quality of the image display to be deteriorated.
That is, in an AS-TFT module for TV use or one capable of coping with an animated picture, 10 to 12 kinds of gradation voltages are supplied to the source driver section 3, such that the optical γ characteristic becomes 2.2. However, due to irregularities in manufacturing the panels, this γ characteristic is also changed, thus causing the display quality to deteriorate.
Here, as examples of a liquid crystal display device which performs γ correction, reference Is made to the following Patent Documents 1, 2.
[Patent Document 1]
Japanese Patent Application (Laid-open) Hei6(1994)-195046 In this Patent Document 1, the following technique is used to perform γ correction in a liquid crystal display device using a ROM.
By inputting the digital image data to an address of the ROM, digital image data having a corrected γ characteristic is obtained from the output of the addressed ROM.
However, the provision of a ROM which has addresses capable of coping with all levels of the digital image data calls for a large capacity ROM. Accordingly, levels of digital image data corresponding to portions which have similar γ characteristics are divided into regions. For example, when the region where the γ characteristic is low is similar to the region where the γ characteristic is high, the respective levels of the low-level region are used as addresses of the ROM and the respective levels of the high-level region are outputted by inverting the output of the ROM at respective levels of the low-level region, thus decreasing the capacity of the ROM.
[Patent Document 2]
Japanese Patent Application (Laid-open) Hei5(1993)-64037
This Patent Document 2 describes the following technique to perform γ correction of a liquid crystal display device automatically using an all-purpose microcomputer.
In an all-purpose microcomputer, γ correction data is stored in a γ correction memory in which image data is inputted, and image data, which has received γ correction and is outputted from the memory, is displayed on the liquid crystal display device. The brightness of the displayed image is measured by a brightness photometer, and the all-purpose microcomputer determines whether or not the measured brightness is a brightness corresponding to the inputted image data, and it stores the γ correction data in the memory for γ correction, such that a corresponding luminance is obtained.